It sometimes happens that electrical conductors must be routed between rotatable portions of an apparatus. For example, in one small folding pocket calculator, that when closed is about one quarter of an inch thick and approximately the length and width of a credit card, the two sections that fold are joined by a hinge. One section carries the display, while the other carries the keyboard and processing means. In such a calculator it is necessary to connect the display to the processing means with several conductors that traverse the hinge. One solution is to simply route the conductors of a flexible printed circuit along the inside surface of the hinge and at right angles to the axis of hinge rotation. The hinge itself may be a strip or flap of flexible material (e.g., cloth), and may allow nearly three hundred and sixty degrees of rotation. With such a solution the radius of the hinging action also essentially determines the bending radius of the conductors. It is frequently desired that the radius of the hinge be small, as any increase in that radius adds to the size of the apparatus. However, as the radius decreases there is an increase in the bending stress incurred by the conductors. Each cycle of rotation applies compression and tension to the conductors. A clear concern in such instances arises for the ability of the conductors to resist breakage after some number of cycles of rotation. A second concern is how to protect the conductors from accidental injury, since in the absence of such protection they are exposed whenever the calculator is opened for use. A common solution in these cases is to simply cover the conductors with a flap of flexible protective material, such as vinyl or cloth. Such a measure is less than foolproof, since the protective flap is itself susceptible to injury, and may be cut or punctured by sharp objects.
As another example, the Hewlett-Packard Model 110 portable computer has a non-articulated hinged display section that rotates about a single axis, so that the display may be unlatched and made visible by rotating it away from the main portion of the computer. The conductors connecting the display section with the main portion of the computer are routed along the axis of rotation in the main portion before entering a hollow hinge. The hinge is formed from two fixed arms that extend from the display section and that rotate about respective distal captive annular sections. The conductors are individual insulated wires in a bundle anchored in a first location by a first strain relief at the point where the bundle enters one of the annular sections of the hinge. Thereafter, the bundle remains fixed in relation to the display as they both rotate together relative to the main body of the computer. Along the bundle in the other direction (i.e., from the strain relief in the display section back toward the main portion of the computer) there is a second strain relief anchoring to the main portion of the computer a second location of that part of the bundle running along the axis of the hinge. At the second location the bundle remains fixed in relation to the main body of the computer. Generally speaking, the first and second locations of strain relief are at opposite ends of that section of the bundle that runs along the axis of hinge rotation. The portion of the bundle between the two strain reliefs thus experiences distributed torsional stress during the rotational excursions of the display section. Such distributed torsional stress is superior to the locally concentrated stresses of compression and tension found in conventional bending. Unfortunately, the hinge and torsional method just described may not be applicable where the amount of rotation desired approaches one complete revolution.
It would be desirable if a pocket calculator or other device could enjoy the benefits of torsional flexing of the conductors connecting rotating parts in an application where the parts were capable of rotating through a complete revolution.
Torsional flexing of conductors, as well as other objects of the invention, may be realized in a design that incorporates a hollow intervening member serving as an articulated hinge between the two parts or assemblies to be interconnected. The term "articulated hinge" means that there are two parallel axes of rotation; a first axis between the intervening member and the casing of a first assembly, and a second axis between the intervening member and the casing of the second assembly. The means that make the articulated hinge captive to the other assemblies does so by having a hollow spindle portion of one part extend a short distance into a corresponding socket portion of another. The end of the hollow spindle portion and the floor of the socket portion each have matching holes that are in alignment, and that are centered about the axis of rotation. In this way an interior passage is formed between the first assembly and the hollow articulated hinge, and then further between the hinge and the second assembly. The result is a well protected interior passage between the first and second assemblies. Several flexible printed circuit traces are routed from one assembly to the other. Each axis of rotation of the articulated hinge is limited to a total displacement of approximately one hundred and eighty to two hundred degrees. The two assemblies can thus move through one complete revolution relative to one another.
The traces of the flexible printed circuits are strain relieved at four places: once upon leaving each assembly to enter the articulated hinge along the associated axis, and twice approximately in the center of the articulated hinge (once on either side of where the flexible printed circuits transition from one axis to the other). The strain relief limits the amount of torsion the flexible printed circuit traces experience at any one place, and apportions the torsion to two separate portions of those traces in accordance with how each assembly is rotated relative to the articulated hinge.
A further object realized by the above-described invention is that of protecting the conductors from damage. This result obtains because the conductors remain fully encased as they go from one assembly to the other.
Still other objects are realized by the invention when the first and second assemblies comprise the top and bottom case halves of a small handheld calculator. These include an easy way to locate keypads on both halves of the case, and the related object of allowing the use of one flexible membrane keyboard assembly to interconnect the two keypads and the processing means.